In the coating of substrates, aqueous binders, in particular polyurethane-polyurea (PU) dispersions, are increasingly being used. In some applications high temperatures, which lead to undesired yellowing of the coating, are required for drying the coatings.
In the area of sizing of glass fibers, PU dispersions are used as aqueous binders. Due to the comparatively high temperatures in the coating and drying processes and in the compounding of the sized glass fibers into a plastics matrix, thermal yellowing of the coatings produced often occurs and is undesired.
The prior art discloses numerous stabilizers and additives which can reduce thermal yellowing of binders. The use of these compounds in aqueous systems, such as PU dispersions, is often possible only to a limited extent. The systems disclosed in the prior art do not adequately meet the needs for protecting an aqueous PU dispersion or the coating resulting therefrom from thermal yellowing.
U.S. Pat. No. 5,216,078 discloses a stabilizer which significantly reduces the thermal yellowing of blocked isocyanates, in particular isocyanates blocked with butanone oxime. This compound is a hydrazine adduct.
EP-A 0 829 500 describes a combination of compounds as a stabilizer for blocked polyisocyanates, one of the compounds having at least one 2,2,6,6-tetramethylpiperidinyl radical, the so-called HALS (hindered amine light stabilizer) radical, and the other having a hydrazide structure.
However, the disadvantage of the abovementioned systems is that they are suitable only for solvent-containing finishes, coatings and sizing systems and not for aqueous systems.
The preparation of aqueous PU dispersions is known in principle. The various possibilities for preparing such dispersions are summarized, for example, by D. Dietrich in a review article (D. Dietrich, Prog. Org. Coatings 9, 281 (1981)). However, the problem of thermal yellowing has not been solved at present in a satisfactory manner in the prior art.
Therefore, it is an object of the present invention to provide PU dispersions which are stable to thermal yellowing and are suitable as one-component or two-component binders in finishes, sizes and coatings.
This object may be achieved with the aqueous PU dispersions according to the invention, which are protected from thermal yellowing by certain combinations of hydrazides and sterically hindered amines.